Ultra wide band dual orthogonal frequency coded SAW correlators using harmonic operation

This paper presents the development of ultra-wideband (UWB) surface acoustic wave (SAW) correlators using harmonic operation for increased center frequency. Orthogonal frequency coding (OFC) is a spread spectrum coding technique with a variety of applications. The OFC SAW correlator spectrally spreads the UWB data beyond that of CDMA due to an increased signal bandwidth. The OFC SAW correlator concept was previously demonstrated using a coded dispersive transducer in conjunction with an apodized transducer in the 250 MHz frequency range. The relatively low frequency was chosen for proof of device concept within the fabrication limitations of conventional contact lithography technology. Due to antenna size considerations, a higher operation frequency is desirable for wireless system operation. Using harmonic device operation, much higher frequency devices are possible while continuing to work within the same limitations of a given lithography technology. Experimental results of a harmonic OFC SAW correlation filter are presented for in-line and multi-tracked device configurations. The performance of the harmonic device design is compared to a device operating at fundamental frequency. The harmonic SAW correlators meet UWB specifications with a fractional bandwidth greater than 29% and were fabricated on lithium niobate (LiNbO₃) having a center frequency in the 1 GHz frequency band. A coupling of modes (COM) model is used to predict the desired SAW filter response experimentally and is compared to the measured data. Good correlation between the predicted COM responses and the measured device data is obtained. Discussion of the design, analysis and measurements are presented. Results are shown for operation in a matched filter correlator for use in an UWB communication system and compared to prediction. The results demonstrate the feasibility of harmonic operation for high frequency SAW OFC correlation filters.